There are known a number of heat-sensitive recording materials of the type that are so designed as to produce a record image when a color former and a color developer are brought into contact with each other by heat. Those heat-sensitive recording materials of this type are comparatively inexpensive and suitable for use in a compact recording machine, which is relatively easy in maintenance. In practice, they are used as a recording medium in facsimile systems, computer systems, heat-sensitive copying systems, and printers of other various instruments.
In recent years, various information instruments have been developed and they have been used in various sectors. For instance, as for the facsimile system, there have been commercialized various facsimile systems of improved high recording speed. There also have been commercialized various video-printers and bar code printers respectively capable of providing high-quality photography-like images. In this respect, there is an increased demand for providing an improved heat-sensitive recording material which is capable of instantly and precisely responding to a slight printing energy to record high quality clear images excelling in gradation and resolution corresponding to information signals transmitted. There is also another increased demand for the heat-sensitive recording material to be used in such instruments that it be so made as to enable it to provide desirable images with a sufficient recording density (optical density) in the entire density range from low density region to high density region, satisfactory gradation and resolution (reproduction of dots).
To meet the above demands, there have been proposed heat-sensitive recording materials having an intermediate layer of an oil absorptive inorganic pigment by Japanese Unexamined Patent Publications 59(1984)-155097 and 61(1986)-44683. There have been also proposed other heat-sensitive recording materials having an intermediate layer of multi-layered structure with an improved smoothness and heat-insulating property by Japanese Unexamined Patent Publications 61(1986)-11286 and 61(1986)-193880. However, any of these heat-sensitive recording materials are still problematic and not sufficient enough to desirably meet the above demands. That is, for the foregoing heat-sensitive recording materials having an intermediate layer containing an oil absorptive inorganic pigment, there are problems that when a high absorptive pigment such as silicon dioxide, calcined kaolin, etc. is used in the case of preparing the former heat-sensitive recording material, the specific surface area with respect to the pigment is increased to cause surface activation of silicic acid and as a result, particularly upon use of an acidic paper as the substrate, the brightness of the remaining white portion not having images thereon is decreased to cause a phenomenon called "background fogginess". Likewise, there are still similar problems also in the case of the foregoing heat-sensitive recording material having an intermediate layer of multi-layered structure. These problems are significant in the case where an acidic paper sheet is used as the substrate. Thus, there is an increased demand to provide an improved heat-sensitive recording material which is free of these problems and complies with the foregoing demands.